In a metal bonding method as described in JP-2001-185843 (corresponding to U.S. Pat. Nos. 6,598,780, 6,601,754, 6,719,187, 6,769,598), when two metal connectors of wirings on two circuit substrates are bonded, the connectors of the circuit substrates are faced each other while inserting therebetween a hydrocarbon compound having a C—H bond dissociation energy of 950 kJ/mol or less. Then, the connectors are heated to thermally decompose the hydrocarbon compound into a hydrocarbon compound radical from which hydrogen has been separated. The resulting hydrocarbon compound radical reduces and thereby removes an oxide film formed on the metal surface constituting the connector. Different from a method using a flux, the oxide film can be removed without forming metal ion.
Metals from which an oxide film has been removed are melted or diffused by heating and can be bonded. In this case, without deteriorating an insulation performance, sufficient bonding strength can be obtained.
As described above, the reduction reaction of an oxide film formed on a metal surface is caused by a hydrocarbon compound radical. It is therefore necessary to form a sufficient amount of a hydrocarbon compound radical for the removal of the oxide film.
In the above-described method, the hydrocarbon compound radical is formed by heating the connectors, thereby thermally decomposing the hydrocarbon compound. The problem of the above-described method resides in that the connectors must be heated to a relatively high temperature in order to form a sufficient amount of the hydrocarbon compound radical. In this case, a highly heat-resistant material must be used as an insulating material of a circuit substrate having a metal connector mounted thereon. A sufficient bonding strength cannot be attained without increasing the heating temperature to, for example, about 400° C. and continue heating for about 1 to 10 seconds.